The purpose of the proposed research is to study the maturation of the intrarenal circulation in the newborn dog. Intrarenal blood flow distribution will be measured by the radioactive microsphere technique. The interrelationships among renal blood flow distribution, arterial blood pressures, glomerular filtration rate and sodium excretion will be evaluated in the developing kidney. Results of the proposed investigation will hopefully answer the question as to whether the inability of the newborn's kidney to adequately respond to a salt stress is related to a different pattern of intrarenal blood flow distribution. The study is also designed to determine whether alterations in the diet of the newborn animal can modify renal maturation.